In recent years, a technique for forming a thin film transistor (TFT) by using a semiconductor thin film (having a thickness of approximately several nanometers to several hundreds of nanometers) formed over a substrate having an insulating surface has attracted attention. Thin film transistors are applied to a wide range of electronic devices such as ICs or electro-optical devices, and prompt development of thin film transistors that are to be used as switching elements in image display devices, in particular, is being pushed. Various metal oxides are used for a variety of applications. For example, indium oxide is a well-known material and used as a material of a transparent electrode which is needed in a liquid crystal display or the like.
Some metal oxides have semiconductor characteristics. Examples of such metal oxides having semiconductor characteristics include tungsten oxide, tin oxide, indium oxide, zinc oxide, and the like. Thin film transistors in which a channel formation region is formed using such metal oxides having semiconductor characteristics are known (Patent Documents 1 and 2).
Moreover, there is a trend in an active matrix semiconductor device typified by a liquid crystal display device towards a larger screen, e.g., a 60-inch diagonal screen, and further, the development of an active matrix semiconductor device is aimed even at a screen size of a diagonal of 120 inches or more. In addition, a trend in resolution of a screen is toward higher definition, e.g., high-definition (HD) image quality (1366×768) or full high-definition (FHD) image quality (1920×1080), and prompt development of a so-called 4K Digital Cinema display device, which has a resolution of 3840×2048 or 4096×2180, is also pushed.
Increase in screen size or definition tends to increase wiring resistance in a display portion. Increase in wiring resistance causes delay of signal transmission to an end portion of a signal line, drop in voltage of a power supply line, or the like. As a result, deterioration of display quality, such as display unevenness or a defect in grayscale, or increase in power consumption is caused.
In order to suppress increase in wiring resistance, a technique of forming a low-resistance wiring layer with the use of copper (Cu) is considered (e.g., see Patent Documents 3 and 4).